Distributed bragg reflector for reflecting light of multiple wavelengths from an LED

ABSTRACT

A blue LED device has a transparent substrate and a reflector structure disposed on the backside of the substrate. The reflector structure includes a Distributed Bragg Reflector (DBR) structure having layers configured to reflect yellow light as well as blue light. In one example, the DBR structure includes a first portion where the thicknesses of the layers are larger, and also includes a second portion where the thicknesses of the layers are smaller. In addition to having a reflectance of more than 97.5 percent for light of a wavelength in a 440 nm-470 nm range, the overall reflector structure has a reflectance of more than 90 percent for light of a wavelength in a 500 nm-700 nm range.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 from U.S.Provisional Application No. 61/530,385, entitled “Distributed BraggReflector for Reflecting Light of Multiple Wavelengths from an LED,”filed on Sep. 1, 2011, the subject matter of which is incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates generally to light-emitting diodes (LEDs),and more particularly, to a blue LED having a reflector structure thatreflects blue and yellow light well.

BACKGROUND INFORMATION

FIG. 1 (prior art) is a simplified cross-sectional diagram of one typeof so-called white LED assembly 1. Assembly 1 includes a lateral blueLED device 2. The active layer 3 of the blue LED device 2 emits light inall directions, and the light bounces randomly within the LED device. Asubstantial amount (about 50%) of light travels downward. If the light 4traveling downwards is not reflected back upward so that it can thenescape from the top surface of LED device, but rather if the lighttraveling downwards is absorbed by the die-attach adhesive or by thealuminum core PCB, then the light generation efficiency of the overallwhite LED assembly will suffer.

The structure of the lateral LED device entails a sapphire substrate 5that is substantially transparent to the blue light. Accordingly, areflector structure 6 is disposed on the backside (i.e., bottom side inthe diagram) of the transparent substrate 5 to reflect light that wastraveling in a downward direction. Reflector structure 6 reflects thelight that travels downwards, passes this light back up and through thetransparent substrate and through the epitaxial layers of the LEDdevice. The reflected light then escapes the LED device and reachesphosphor 7 embedded in encapsulant, such as silicone. The phosphorabsorbs some of the blue light and fluoresces, thereby re-emitting lightof longer wavelengths including green, yellow and red light. The overallspectrum of light emitted from the overall LED assembly 1 is thereforesaid to be white light. This white light is the useful light produced bythe assembly.

The reflector structure 6 can be a single layer of a highly reflectivemetal such as, for example, silver. Unfortunately, silver has attendantcontamination and electromigration issues. For this and other reasons,LED devices such as the LED device 2 of FIG. 1 may have reflectorstructures involving a total internal reflection (TIR) layer 8, aDistributed Bragg Reflector (DBR) structure 9, and an underlying layer10 of reflective metal. The combination of these layers is superior interms of reflectivity to a single mirror layer of a highly reflectivemetal.

According to Snell's law, all of the light traveling from a materialhaving a higher index of refraction toward a material having a lowerindex of refraction at an angle greater than the critical angle will bereflected back into the higher-index-of-refraction material withoutexperiencing any energy loss. This mechanism is known as total internalreflection (TIR). The TIR layer 8 is fashioned to reflect blue lightthat is passing toward the reflector at angles greater than the criticalangle. The lower two portions 9 and 10 of the reflector structure (theDBR and the reflective metal layer) are provided to reflect anyremaining light that passes through the TIR layer.

In its simplest form, a DBR is a quarter wave stack of dielectricmaterials. The quarter wave stack consists of a stack of layers, wherethe material from which the layers are made alternates from layer tolayer down the stack. The materials are selected such that thealternating layers have a high index of refraction, and then a low indexof refraction, and then a high index of refraction, and so forth downthe stack. For a given wavelength of light entering the stack from thetop, the upper layer is made to have a thickness of one quarter of thewavelength, where this wavelength is the wavelength of the light whenthe light is passing through the layer. The wavelength λ, frequency f,and velocity v of light is given by the equation λ=v/f. When lightleaves one medium and enters another medium, the speed and wavelength ofthe light may change but the frequency does not change. The materialfrom which the upper layer is made therefore determines the speed oflight v in the medium. The material therefore also influences thewavelength λ of the light in the upper layer.

Each material has an index of refraction η. The index of refraction η isthe ratio of the speed of light in a vacuum to the speed of light in themedium. The wavelength of light in a medium is given by the equationλ=λo/η, where λo is the wavelength in a vacuum. Light traveling throughair is traveling at close to the speed of light in a vacuum, so thewavelength of light in air is close to wavelength of the light in avacuum. The design wavelength λo for the DBR is usually longer than theLED emitting wavelength when the reflectivity of the DBR for the lightwith incident angles between zero degrees and the critical angle isconsidered. For example, the optimal DBR design wavelength for a 450 nmLED is around 510 nm. The relationship QWOT=λo/4η is used to determinethe quarter wavelength in the medium of a layer, where η is therefractive index of the material from which the layer is made. In thisway, the refractive indices of the materials of the various layers ofthe stack are used to determine how thick each layer of the stack shouldbe so that it is one quarter wavelength in thickness.

Light passes into the stack and through the upper layer, and then someof the light reflects off the interface between the upper layer and thenext layer down in the stack. Part of the light proceeds down into thenext layer of the stack to the next interface. If the interface is onefrom a low-index medium to a high-index medium, then any light reflectedfrom the interface will have a phase shift of 180 degrees. If, on theother hand, the interface is one from a high-index medium to a low-indexmedium, then any reflected light will have no phase shift. Eachinterface causes a partial reflection of the light wave passing into thestack. The phase shifts, in combination with the thicknesses of thelayers of the stack, are such that the portions of light reflecting offinterfaces all return to the upper surface of the stack in phase witheach other. The many reflections off the many interfaces all combine atthe top of the stack with constructive interference. The result is thatthe Distributed Bragg Reflector has a high reflectivity within a finitespectral range known as the stop-band. Then lastly at the bottom of thereflector structure 6 is the layer 10 of reflective metal.

FIG. 2 (prior art) is a table that sets forth the thicknesses andmaterials of the various layers of the Distributed Bragg Reflector ofthe prior art LED device 2 of FIG. 1 based on a design wavelength of 510nm. The n notation above the line between two rows indicates that thelight reflected by the interface between the materials of the two rowsis phase shifted by 180 degrees. The upper SiO₂ layer has a thickness of4101 angstroms and is the TIR layer 8. The DBR structure 9 includesthree periods, where each period has a first layer of TiO₂ that is 447angstroms thick and a second layer of SiO₂ that is 820 angstroms thick.

FIG. 3 is a diagram that shows the normal-incident reflectivity spectrumwith the reflector design described in FIG. 2. The stop-band of thespectrum centers around 510 nm, and the short wavelength side of thestop-band is aligned to 450 nm. According to theoretical calculation,the reflectivity spectrum blue-shifts toward the short wavelength whenthe light incident angle increases from surface normal toward grazingangle to the reflector. The reflector was optimized to ensure highreflectivity for the light with wavelength of 450 nm over a broad rangeof incident angles. FIG. 4A is a diagram that charts the reflectivity ofthe reflector structure 6 versus the angle of incidence of light with awavelength of 450 nm reaching a point 11 on the reflector. The lightwith incident angles between 0 and 58 degree are reflected by the DBRand the metal reflector, while the light with incident angle greaterthan 58 degree is reflected by the TIR layer. To evaluate the totalreflectivity of the reflector with all incident angles, a normalizedangular reflectance is defined. Referring to FIG. 4B, light is assumedto be transmitted toward point 11 on the reflector from all directionswith a uniform angular distribution. The amount of light incident on thepoint that is reaching the point 11 with an incident angle θ isconsidered. Many different light rays may actually reach the point fromthis incident angle, where the light rays can be thought of as passingto the point in a cone shape. The upper lip of the cone 12 illustratedin FIG. 4B represents a circle of origination points for such rays forthe incident angle θ.

Accordingly, there is more light incident on point 11 for an incidentangle of one degree than for an incident angle of zero degrees. Thislarger amount of light at larger angles is considered, and thecorresponding total amount of reflected light is determined for angleszero (orthogonal) through 90 degrees (a grazing angle). The normalizedangular reflectance is then calculated by integrating the angularreflectivity (FIG. 4A) with a sine dependence of incident angle andnormalized to a perfect angular reflectivity spectrum. This analysis isperformed for light of a given wavelength, for example 450 nm, tocompare the performance of the reflector for blue light emitted by theLED in the white LED assembly FIG. 1. When analyzed this way, the priorart reflector structure of the LED device of FIG. 1 has a reflectivityof approximately 97 percent for incident blue light (having a wavelengthof 450 nm). Accordingly, most all of the blue light 4 traveling downwardis then reflected back up the reflector so that it can escape the LEDdevice. The reflector structure involving DBR 9 is more effective than asimple mirror layer of a reflective metal such as silver.

SUMMARY

A blue LED device has an active layer involving indium, gallium andnitrogen. The active layer is configured to emit blue light that isquasi-monochromatic and non-coherent. The blue LED also has atransparent substrate (substantially transparent to visible light) and areflector structure disposed on the backside of the substrate. Thereflector structure includes a Distributed Bragg Reflector (DBR)structure having layers configured to reflect green, yellow and redlight as well as blue light. In one example, the DBR structure includesa first portion where the thicknesses of the layers are relativelylarger, and also includes a second portion where the thicknesses of thelayers are relatively smaller. In addition to having a normalizedangular reflectance of more than 97.5 percent for light of a wavelengthin a first range between 440 nm-470 nm, the overall reflector structurealso has a normalized angular reflectance of more than 95 percent forlight of a wavelength in a second range between 500 nm-700 nm. Thereflector structure reflects light passing from the transparentsubstrate and to the reflector structure such that the overall LEDdevice has a Photon Recycling Efficiency (PRE) of more than 85 percentfor light having a wavelength ranging from 500 nm to 700 nm.

Further details and embodiments and techniques are described in thedetailed description below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 (prior art) is a simplified cross-sectional diagram of aconventional so-called white LED.

FIG. 2 (prior art) is a table that sets forth the thicknesses andconstituent materials of the various layer of the Distributed BraggReflector of the prior art LED device of FIG. 1.

FIG. 3 (prior art) is a diagram that charts reflectivity versus thewavelength of incident light at a normal incident angle for thereflector structure of the prior art LED device of FIG. 1.

FIG. 4A (prior art) is a diagram that charts the reflectivity of thereflector structure of the prior art LED device of FIG. 1 versus theangle of incidence of light of a wavelength of 450 nm reaching a pointon the reflector.

FIG. 4B (prior art) is a conceptual diagram that illustrates aconsideration involved in determining the normalized angularreflectance.

FIG. 5 is a diagram of a white LED assembly in accordance with one novelaspect.

FIG. 6 is a simplified cross-sectional diagram of a blue LED devicewithin the white LED assembly of FIG. 5.

FIG. 7 is a table that sets forth the thicknesses and constituentmaterials of the various layers of the novel reflector structure ofFIGS. 5-6.

FIG. 8 is a diagram that charts reflectivity versus wavelength ofincident light normal to the reflector surface for the novel reflectorstructure of FIGS. 5-7.

FIG. 9 is a table that compares the normalized angular reflectance at450 nm and at 580 nm of the novel reflector structure of FIGS. 5-7 tothe prior art reflector structure of FIGS. 1-4.

FIG. 10 is a table that compares measured PRE values of the novelreflector structure of FIGS. 5-7 (at 450 nm, 580 nm, and 630 nm) withcalculated PRE values to the prior art reflector structure of FIGS. 1-4(at 450 nm, 580 nm, and 630 nm).

FIG. 11 is a flowchart of a method for forming a reflector structure ona blue LED that exhibits a high normalized angular reflectance for lighthaving a wavelength in a range from 500 nm to 700 nm and in anotherrange from 440 nm to 470 nm.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 5 is a simplified cross-sectional diagram of a white LED assembly20 in accordance with one novel aspect. White LED assembly 20 includes ablue LED device 21, an aluminum core PCB 22, a pair of wire bonds 23 and24, and an amount of phosphor 25. Particles of phosphor are suspended ina dome structure of silicone as illustrated. LED device 21 includes anepitaxial layer portion that includes, among other parts notillustrated, a p-type layer 26, an active layer 27, an n-type layer 28,a buffer layer 29, and two metal electrodes 30 and 31. The layers 26-28are made of gallium nitride materials and the active layer includesindium such that the active layer emits so-called blue light as is knownin the GaN blue LED art. The light is quasi-monochromatic andnon-coherent. In the present example, the wavelength of the lightemitted by the active layer 27 has a relatively narrow bandwidth and iscentered at approximately 450 nm.

The epitaxial layers are disposed on a transparent substrate 32.Transparent substrate 32 is made of a transparent material, such assapphire, SiC, GaN or AlN. In the present example, the transparentsubstrate 32 is a sapphire substrate. Below substrate 32 is a novelreflector structure 34. Reflector structure 34 includes a total internalreflection (TIR) layer 35, a multi-layer Distributed Bragg Reflector(DBR) structure 36, and a reflective metal layer 37. TIR layer 35 andthe low refractive index layers of DBR 36 can be made of low indexdielectric material, such as SiO2, MgF2 or CaF2, and the high indexlayers of DBR 36 can be made of high index dielectric material, suchTiO2, ZnSe, Si3N4, Nb2O5 or Ta2O5. Reflective metal layer 37 can be madeof any reflective metal, such as aluminum, silver, rhodium, platinum ornickel. Reflector structure 34 is disposed on the “backside” of thesubstrate on the opposite side of the substrate from the epitaxiallayers. FIG. 6 is a more detailed cross-sectional diagram of the blueLED device 21 of the white LED assembly 20 of FIG. 5.

As is conventionally recognized, half of the light emitted from theactive layer of an LED travels downward. This light, which in thepresent example has a wavelength of approximately 450 nm, should bereflected back upward by the reflector structure as described above inthe background section. This light is represented in FIG. 5 by rays 38and 39.

In accordance with one novel aspect, it is now recognized that some ofthe light 40 traveling upwards escapes the LED device and reaches thephosphor 25 but is then down-converted by the phosphor into light oflonger wavelengths. Some of this converted light 41 then travels backtowards the LED device in such a way that it passes into the LED device.The light that is emitted back at the LED device by the phosphor isgenerally in the range of from 500 nm to 700 nm and is referred to herefor simplicity purposes as “yellow” light. This light is represented inFIG. 5 by rays 41-42. Whereas in the prior art reflector structuredescribed above in connection with FIGS. 1-4 the reflector structure wasnot optimized to reflect light of this yellow wavelength, the novelreflector structure 34 of FIG. 5 is designed to improve the reflectivityof light of this wavelength. The novel reflector structure 34 is notoptimized for reflecting only blue light, and is not optimized forreflecting only yellow light, but rather the layers of the novelreflector structure are configured to reflect both blue and yellow lightwith high reflectivity. Thus, the novel reflector structure 34 has a DBRthat is substantially optimized for reflecting both blue light ofapproximately 450 nm and yellow light of approximately 580 nm. In oneexample, the reflector structure 34 has a normalized angular reflectanceof more than 95.5 percent for first light having a wavelength in a rangefrom 500 nm to 700 nm (referred to here as yellow light), and also has anormalized angular reflectance of more than 97.5 percent second lighthaving a wavelength in a range from 440 nm to 470 nm (referred to hereas blue light). The photon efficacy (lumens per watt) of the overallnovel LED assembly 20 of FIG. 5 is improved as compared to the photonefficacy of the overall conventional LED assembly 1 of FIG. 1 largelydue to the improved reflectivity of the reflector structure 34 inreflecting the light in the 500 nm to 700 nm range.

Designing the DBR structure 34 is not as simple as designing a first DBRoptimized for reflecting yellow light, and designing a second DBRoptimized for reflecting blue light, and then combining the two DBRsinto a single composite DBR structure. Light passing through the DBRstructure from one portion to the next is affected in complex ways thatcomplicates the determination of the thicknesses of the various layers,and the DBR is not entirely optimized for either yellow or blue light,but in a simplistic explanation a first portion 43 of the DBR 34functions primarily to reflect yellow light, whereas a second portion 44of the DBR 34 functions primarily to reflect blue light. The thicknessesof the layers of the first portion 43 are larger, whereas thethicknesses of the layers of the second portion 44 are smaller.

FIG. 7 is a table that sets forth the thicknesses and compositions ofthe various layers of the reflector structure 34 in one specificembodiment. Row 45 corresponds to the TIR layer 35. Rows 46 correspondto the first portion 43 of the DBR structure 36, and rows 47 correspondto the second portion 44 of the DBR structure 36. Row 48 corresponds tothe layer 37 of reflective metal. The values in the table are for adesign wavelength of 480 nm. Accordingly, the quarter-wave opticalthickness (QWOT) values close to one in rows 47 indicate that the secondportion 44 of the DBR structure will reflect blue light well.

FIG. 8 is a chart of the reflectivity 49 versus wavelength for a normalincident angle for the overall reflector structure 34. The chartcompares the reflectivity spectrum of the prior art reflector to that ofthe novel reflector. There are two distinct stop-band features for thenovel reflector indicating the complexity of the reflector design.Dashed curve 50 is the reflectivity versus wavelength curve 50 of FIG. 3that is reproduced in FIG. 8 for comparison purposes.

FIG. 9 is a table that sets forth the comparison. For first light havinga wavelength of 580 nm (generally referred to herein as yellow light)passing from the substrate and into the reflector structure, the novelreflector structure 34 of FIGS. 5-7 has a reflectivity greater than 95.0percent. For second light having a wavelength of 450 nm (generallyreferred to herein as blue light) passing from the substrate and intothe reflector structure, the novel reflector structure 34 of FIGS. 5-7has a reflectivity greater than 97.5 percent.

Referring to white LED assembly 20 of FIG. 5, the phosphors absorb theblue light emitted from the LED device 21 and down-convert it to longerwavelength (500 nm-700 nm) light. The long wavelength light re-emittedisotropically from the phosphor particles and some portion of longwavelength light will inevitably return to the LED surface. Theprobability of the returned light to escape the LED device 21 isreferred as the Photon Recycling Efficiency (PRE). The un-absorbed bluelight emitted from the LED device may also be back-scattered by thephosphors and return to the LED device. A comprehensive ray-tracingmodel was employed to estimate the PRE for various wavelengths light.The absorption of the Indium Tin Oxide (ITO), the metal electrode, GaNmaterial loss, the scattering structure and the reflector were allincluded in the simulation.

The simulation was performed using 450 nm light, 580 nm light, and 630nm light. The percentage of light reflected (or “PRE”) is set forth inthe table of FIG. 10. The relatively small differences in reflectivitybetween the novel reflector structure and the conventional reflectorstructure indicated in the table of FIG. 9 are amplified in the realdevice due to light within the LED device often making multiple bounceswithin the device. Simulation indicates that switching from theconventional reflector structure 6 of FIG. 1 to the novel reflectorstructure 34 of FIG. 5 results in more than a 5.0 percent improvement inPhoton Recycling Efficiency for both 580 nm light and 630 nm light.

FIG. 11 is a flowchart of a method 100 in accordance with one novelaspect. A reflector structure is formed (step 101) on the backside of asubstrate of a blue LED device. The active layer of the blue LED deviceis configured to emit light having a wavelength of approximately 440-470nm, whereas the reflector structure has a normalized angular reflectancegreater than 95.0% for light having a wavelength in a range from 500 nmto 700 nm. In one specific example, the reflector structure also has anormalized angular reflectance greater than 97.5% for light having awavelength of 440-470 nm. In one specific example, the reflectorstructure formed in step 101 is the reflector structure 34 of FIGS. 5and 6, where this reflector structure 34 has a TIR layer, a DBRstructure, and an underlying layer of metal of the thicknesses andconstituent materials set forth in FIG. 7.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. Accordingly, various modifications, adaptations, andcombinations of various features of the described embodiments can bepracticed without departing from the scope of the invention as set forthin the claims.

What is claimed is:
 1. A Light Emitting Diode (LED) device comprising: asubstrate; and a reflector structure disposed below the substrate, thereflector structure comprising: a low-index total internal reflectionlayer (TIR) disposed below the substrate; a Distributed Bragg Reflector(DBR) disposed below the TIR; and a reflective metal layer disposedbelow the DBR, wherein the DBR comprises a first plurality of periodsand a second plurality of periods, wherein each of the first pluralityof periods includes a first layer of a high index dielectric materialwith a first thickness and a second layer of silicon dioxide with asecond thickness, and wherein each of the second plurality of periodsincludes a first layer of the high index dielectric material with athird thickness and a second layer of silicon dioxide with a fourththickness, wherein the TIR is a single layer of silicon dioxide that isthicker than any silicon dioxide layer of the DBR.
 2. The LED device ofclaim 1, wherein the high index dielectric material is selected from thegroup consisting of: TiO2, ZnSe, Si3N4, Nb2O5 and Ta2O5.
 3. The LEDdevice of claim 1, wherein the reflector structure has a reflectancegreater than 90 percent for first light passing from the substrate tothe reflector structure, and wherein the first light has a wavelength ina first range from 500 nm to 700 nm.
 4. The LED device of claim 3,wherein the reflector structure has a reflectance greater than 90percent for second light passing from the substrate to the reflectorstructure, and wherein the second light has a wavelength in a secondrange from 440 nm to 470 nm.
 5. The LED device of claim 1, wherein thefirst layer of each of the first plurality of periods is titaniumdioxide approximately 75 nm thick, wherein the second layer of each ofthe first plurality of periods is silicon dioxide approximately 138 nmthick, wherein the first layer of each of the second plurality ofperiods is titanium dioxide approximately 46 nm thick, and wherein thesecond layer of each of the second plurality of periods is silicondioxide approximately 85 nm thick.
 6. The LED device of claim 5, whereina titanium dioxide layer of the first plurality of periods of the DBR isin contact with the TIR.
 7. The LED device of claim 5, wherein atitanium dioxide layer of the second plurality of periods of the DBR isin contact with a silicon dioxide layer of the first plurality ofperiods.
 8. The LED device of claim 1, wherein the reflective metallayer is made of a metal taken from the group consisting of: aluminum,silver, rhodium, platinum and nickel.
 9. The LED device of claim 1,wherein the substrate is a transparent substrate.
 10. The LED device ofclaim 1, wherein the second thickness is greater than the firstthickness, and wherein the fourth thickness is greater than the thirdthickness.
 11. The LED device of claim 10, wherein the first high indexdielectric material is selected from the group consisting of: TiO2,ZnSe, Si3N4, Nb2O5 and Ta2O5; and wherein the second high indexdielectric material is selected from the group consisting of: TiO2,ZnSe, Si3N4, Nb2O5 and Ta2O5.
 12. The LED device of claim 1, furthercomprising an active layer configured to emit a first light of awavelength less than 500 nm, wherein the reflector structure has areflectance greater than 90.0 percent for a second light passing fromthe substrate and to the reflector structure, and wherein the secondlight has a wavelength in a range from 500 nm to 700 nm.
 13. The LEDdevice of claim 1, further comprising an active layer comprising indiumand gallium and configured to emit light of a wavelength less than 500nm, wherein an overall LED device exhibits a Photon Recycling Efficiency(PRE) of more than 85 percent for light having a wavelength in a rangeof 500-700 nm.
 14. The LED device of claim 1, further comprising anactive layer configured to emit a first light of a wavelength ofapproximately 440-470 nm, wherein the reflector structure has areflectance greater than 90.0 percent for a second light passing fromthe substrate to the reflector structure, and wherein the second lighthas a wavelength of approximately 500-700 nm.